Phosphors for fluorescent lamps [low pressure mercury vapour (Ipmv) lamps, excited by mercury discharge at low pressure corresponding to radiation of wavelength 254 nm] range from the conventional halophosphates [Ca.sub.5 (PO.sub.4).sub.3 F:Sb.sup.3+, Mn.sup.2+ ; with a color rendering index (CRI).about.70 and an efficacy 65 lm/W], to tricolor blend of rare earth phosphors (with an improved CRI of 90 and an efficacy .about.85 lm/W). The CRI of halophosphates can be increased without altering the lumen output by adding a blue emitter (broad band) to it, which covers the whole of the blue region. The phosphor Ca.sub.5 (PO.sub.4).sub.3 F:Sb (emission maximum at 480 nm) satisfies this requirement. In the case of high efficiency tricolor (trichromatic) fluorescent lamps based on rare-earth phosphors, where a blend of three different inorganic compounds each emitting in different regions viz., blue (450 nm), green (540 nm) and red (610 nm) mixes upon to give out white light, when excited by mercury discharge at low pressure corresponding to radiation of wavelength 254 nm, further improvement of CRI is not a serious requirement. Still higher value of CRI (95) is achieved in the case of super-deluxe lamps (efficacy only 65 lm/W). This is possible by having a blend of a broad band emitting red with a broad band emitting blue as well as a line emitting green. The phosphor Sr.sub.4 Al.sub.14 O.sub.25 :Eu.sup.2+ (emission maximum at 490 nm) and Sr.sub.2 P.sub.2 O.sub.7 :Eu.sup.2+ (emission maximum at 420 nm) act as blue components in deluxe lamps. In general, narrow band (or line) emitters help to increase the efficacy while broad band emitters increase the CRI in Ipmv lamps. The requirements of all these components for use in lpmv lamps are mainly strong emission in the required regions when excited with 254 nm wavelength and thus strong absorption of 254 nm radiation. In addition, it must be easily synthesizeable and must be stable [the inorganic dopant(s) present in the phosphor should not undergo any change in its valence state when heated to high temperatures .gtoreq.600.degree. C., one of the essential conditions required during the process of lamp manufacturing (baking), and the phosphor should not degrade at ordinary and at high temperatures (.gtoreq.900.degree. C.) and on continuous irradiation by light of wavelength 254 nm]. Hitherto, the blue components have been either calcium tungstate (CaWO.sub.4, emission at 420 nm) or calcium halophosphate (Ca.sub.5 (PO.sub.4).sub.3 F) doped with antimony (Sb.sup.3+) in the case of conventional halophosphate system, or barium magnesium aluminate (BaMgAl.sub.10 O.sub.17) and strontium chloroapatite (Sr.sub.5 (PO.sub.4).sub.3 Cl) doped with divalent europium (Eu.sup.2+) in the tricolor phosphor blend, or strontium aluminate (Sr.sub.2 Al.sub.6 O.sub.11) and strontium pyrophosphate (Sr.sub.2 P.sub.2 O.sub.7) doped with divalent europium (Eu.sup.+) in deluxe lamps. Eventhough, these phosphors emit in the required region with high intensity, the phosphates and aluminates require very high temperatures (.gtoreq.1200.degree. C. for phosphates and .gtoreq.1500.degree. C. for aluminates) for their synthesis. In addition, they require a strong reducing atmosphere (N.sub.2 :H.sub.2 or H.sub.2 gas flow) to reduce Eu.sup.3+ to Eu.sup.2+, at that temperatures.
In the case of cathode-ray phosphors used in conventional televisions (CTV), the phosphor ZnS:Ag.sup.+ is the blue component. Here, the phosphor needs to get excited with long wavelength UV (355 nm) which is compatible with the cathode-ray excitation. For the beam-index phosphor used in television tubes, a short decay time is necessary to avoid afterglow or persistence. The life time of an electron in the excited state of Ce.sup.3+ ion, when doped in any inorganic crystal lattice is very short. Hence, the Ce.sup.3+ ion-doped lattices are preferred to avoid afterglow. Eventhough the silicate phosphors Y.sub.2 Si.sub.2 O.sub.7 :Ce.sup.3+ (emission maximum at 375 nm) and Y.sub.2 SiO.sub.5 :Ce.sup.3+ (emitting in the blue region) are known as beam-index phosphors, the silicate phosphors not only require very high temperatures for their synthesis but also require repeated firing at high temperatures with several intermittent grindings.